Condensation-evaporator nanoparticle charger

ABSTRACT

A particle charging method and apparatus are provided. An ion source is applied to a particle laden flow. The flow is introduced into a container in a laminar manner. The container has at least a first section, a second section and a third section. The first section includes wetted walls at a first temperature. A second section adjacent to the first section has wetted walls at a second temperature T 2  greater than the first temperature T 1 . A third section adjacent to the second section has dry walls provided at a temperature T 3  equal to or greater than T 2 . Additional water removal and temperature conditioning sections may be provided.

CLAIM OF PRIORITY

This application is a continuation in part of U.S. patent applicationSer. No. 13/218,393 filed on Aug. 25, 2011 entitled, “ADVANCED LAMINARFLOW WATER CONDENSATION TECHNOLOGY FOR ULTRAFINE PARTICLES”, whichclaims priority to U.S. Provisional Application No. 61/402,348 filedAug. 27, 2010, which applications are hereby incorporated by referencein their entirety.

This application claims priority to U.S. Provisional Application Ser.No. 61/709,949 filed Oct. 4, 2012, inventors Susanne V. Hering, StevenR. Spielman, Gregory S. Lewis, which application is hereby incorporatedby reference in its entirety.

GOVERNMENT LICENSE RIGHTS

This technology was made with government support under Grant No.DE-SC0004643 and DE-SC00009644 from the US Department of Energy. Thegovernment has certain rights in the technology.

BACKGROUND

Electrostatic deposition and electrical mobility size separation ofairborne particles are widely used techniques for the collection oranalysis of airborne particles. These methods require that the particlesto be collected or analyzed carry an electric charge. However, for verysmall particles with diameters less than about 50 nm, adding anelectrical charge is difficult. In this size range exposure to a bipolarion source provides singly charged particles, but the chargingefficiency is low. For particles with diameters of 50 nm, just 17% ofthe particles will acquire a positive charge, with an approximatelyequal number acquiring a negative charge. At 10 nm the fraction ofparticles charged with a single polarity is ˜4%, and at 3 nm this dropsto less than 2%. Unipolar charging can improve charging efficiencies forparticles above about 10 nm, but it also becomes ineffective at smallerparticle sizes.

One technique that has been used to increase the charging efficiency ofthese small particles is condensation-enhanced particle charging,wherein the particles are grown through condensation, charged andre-evaporated. Some prior art techniques have used butanol condensationto prepare highly charged particles in the 10-30 nm size range. Othershave used condensation of glycol to enhance the charging of sub-20 nmparticles. Still others have explored this approach with watercondensation, albeit for larger (80-130 nm) particles. Limitations ofthese existing methods are: (1) the contamination of the particlethrough the use of organic materials as the condensing vapor, (2)addition of multiple electrical charges to each particle, and (3)inability to charge particles below about 10 nm.

SUMMARY

A system and method to provide efficient, low-level electrical chargingof particles in the sub-100 nm size range is disclosed. This method usesan ion source coupled to a laminar flow water condensation andevaporation cell. Ions are introduced together with a particle-ladenflow into a water condensation and evaporation device. In the presenceof the ions, particles grow through water condensation, collide with theions to become charged, and then quickly evaporate to return theparticle to near its original size. The dried particle retains theelectrical charge acquired as a droplet, leaving a higher fraction ofcharged particles than entered the system. The time as a droplet can beshort, less than 200 milliseconds. With this short residence time theopportunities for chemical artifacts are minimized. The process occursin a laminar flow, wherein the saturation ratios can be controlled, andcalculated.

A particle charging method and apparatus are provided. An ion source isapplied to a particle laden flow. The flow is introduced into acontainer in a laminar manner. The container has at least a firstsection, a second section and a third section. The first sectionincludes wetted walls at a first temperature. A second section adjacentto the first section has wetted walls at a second temperature T2 greaterthan the first temperature T1. A third section adjacent to the secondsection has dry walls provided at a temperature T3 equal to or greaterthan T2. Additional water removal and temperature conditioning sectionsmay be provided.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a nanoparticle condensation charger inaccordance with the present technology.

FIG. 2 is a graph of the calculated saturation ratio (top) and dropletdiameter (bottom) as a function of axial position for a system operatedto produce saturations sufficient to activate 3 nm particles.

FIG. 3 shows a configuration of the nanoparticle condensation chargerwith two additional stages added for water vapor removal and temperaturerecovery.

FIG. 4A illustrates a system condensation-evaporator for thenanoparticle charger.

FIG. 4B is a cross sectional view of the system of FIG. 4A, showing twoof the three parallel tubes used for particle growth and evaporation.

FIG. 5A shows the typical wall temperatures used in the operation andtesting of the system of FIG. 4A.

FIG. 5B shows, for the scenario of FIG. 5A, the saturation ratioscalculated for flow trajectories along the centerline.

FIG. 5C shows, for the scenario of FIG. 5A, the dew point valuescalculated for flow trajectories along the centerline.

FIG. 6 illustrates the experimental configuration used to measure thecharging efficiency and charge distribution produced by acondensation-evaporator nanoparticle charger.

FIG. 7A shows the mobility distribution output by the nanoparticlecharger of the configuration shown in FIG. 1 using a bipolar ion source,when presented with a test aerosol is centered at 25 nm.

FIG. 7B shows the mobility distribution obtained with the system of FIG.7A for an input aerosol centered at 10 nm.

FIG. 8A shows the mobility distribution output by the nanoparticlecharger of the configuration shown in FIG. 4 using a bipolar ion source,when presented with a test aerosol is centered at 20 nm. Singly charged20 nm particles appear at a mobility size of 20 nm, while multiplycharged particles, being more mobile, appear at smaller mobilitydiameters. The mobility distribution for simple, bipolar charging isalso shown.

FIG. 8B shows the mobility distribution obtained with the system of FIG.8A for an input aerosol centered at 10 nm.

DETAILED DESCRIPTION

Technology is provided for the placement of electrical charge onultrafine, airborne particles. The condensation-evaporator nano-particlecharging technology described herein places a controlled electricalcharge on ultrafine and nanometer sized particles, generally those withdiameters in the range from a few nanometers to a few hundrednanometers. This charging method uses an ion source in conjunction witha laminar flow, water condensation and droplet evaporator system. Theion source can be either a unipolar source, such as created throughcorona wire discharge, or a bipolar source, such as obtained withradioactive sources or soft-x-rays. The condensation-evaporator systemis a multistage device with a two stage condenser as described in U.S.patent application Ser. No. 13/218,393, and which is specificallyincorporated herein by reference.

With reference to FIG. 1, the particle-laden airflow 110 passes throughthe ion source 125, and into the condensation-evaporator system 100.Ions are added to the particle laden flow by the ion source, and carriedwith the flow 111 into the condensation-evaporator. This ion source maybe a bipolar source, such as is achieved with soft x-Rays or with aPo-210 or Kr-85 source. It could also be a unipolar ion source, or aflow with a high concentration of ions that is mixed with the flow 110.System 100 includes a first stage 120 generally referred to as a“conditioner.” The second stage 130 and third stage 140, referred to asthe “initiator” and “evaporator”, respectively, form a two-partcondenser, as described in U.S. patent application Ser. No. 13/218,393.The conditioner 120 is generally operated with slightly cooled walls,and is used to condition the flow 110 to near the temperature of theconditioner walls, with a relative humidity near 100%. The second,“initiator” stage 130 has walls which are maintained warmer than that ofthe conditioner 120. The third, evaporator stage 140 is operated warmerthan the initiator stage 130. As the cooler flow from the conditionerenters the warm, wet walled initiator section, water vapor diffuses fromthe walls into the cooler flow. Likewise the flow slowly warms. Yet,because of its high diffusion constant relative to the thermaldiffusivity of air, water vapor diffuses more quickly. As a result, theflow becomes supersaturated, with its peak supersaturation along thecenterline of the flow.

Particles larger than a certain size grow through condensation of watervapor to form droplets. Typically, this size is in the range of 3 nm to10 nm. The droplets that are formed are several hundred nanometers indiameter. Ions that have been carried with the flow 111 attach to thedroplet-encapsulated particles, creating an electrically chargeddroplet. Because the ion attachment is a strong function of particlesize, the ion attachment to the droplets is much more efficient thanwere the particles not enlarged through condensation. Once charged, thedroplets are evaporated by lowering the relative humidity in the flow toless than 100%. Experimental data shows that upon evaporation, theparticles return to near their original size while retaining theelectrical charge acquired as droplets. By operating the condensationsystem at saturation ratios in excess of 1, for example in the range of1.2 to 1.8, it is possible to activate condensational growth onparticles as small as 3 to 10 nm in diameter. For these small particles,the condensation-evaporation system facilitates much more efficientproduction of charged particles than is possible through direct exposureto an ion source.

The condensation-evaporator system illustrated in FIG. 1 consists of aconditioner 120, an initiator 130 and an equilibrator 140. The walls ofthe conditioner 120 and of the initiator 130 are actively wetted, as canbe done using a wetted wick material lining the walls. The temperatureT1 of the walls of the conditioner 120, is lower than the temperature T2of the walls of the initiator 130. The walls of the equilibrator 140 aredry, and held at temperature T3 which is higher than, or equal to T2.The geometry of the system can be cylindrical, or it can consist ofparallel plates. The flow enters the conditioner 120, and then flowsthrough the initiator 130 and evaporator 140. The volumetric flow rateis constrained to producing a predominantly laminar flow. Apredominantly laminar flow for a cylindrical geometry means the flowReynolds number is generally below 2000. Additionally, the volumetricflow is sufficient to minimize buoyancy effects, corresponding to avalue of the Froude number greater than 1. The Froude number describesthe relative magnitude of forced to buoyancy-driven convection, and isdefined by Fr=(ρV²)/(Δρg L), where V is the characteristic velocity forforced convection, Δρ is the change in air density due to temperaturedifference, g is the gravitational constant and L is the characteristicdistance. For a cylinder, this characteristic distance scales as thetube radius. For flow through a tube of the order of 1 to 2 L/min, witha temperature difference between successive stages of less than 50° C.,these criteria can both be met through use of tube diameters of lessthan about 0.7 cm.

Again with reference to FIG. 1, the conditioner 120 serves to bring thetemperature of the flow 111 entering the initiator 130 to a known value,and to regulate the relative humidity to be at, or near 100% at theconditioner wall temperature. The initiator 130 that follows theconditioner 120 is generally a shorter length than the conditioner, withwalls that are warmer than those of the conditioner 120. Like theconditioner 120 the walls of the initiator 130 are also wetted. Theevaporation of water from the wetted walls of the initiator 130 suppliesthe water vapor necessary to create supersaturation necessary forparticle activation. Because the entering flow is cooler than theinitiator wall temperature, and because water vapor diffuses morequickly than the carrier gas (generally air), the flow becomessupersaturated. This supersaturation activates the condensational growthon small particles, as described by U.S. patent application Ser. No.13/218,393. However, the initiator does not provide sufficient time fordroplet growth. Most of the droplet growth and the droplet evaporationoccur in the warm, dry-walled “evaporator” stage 140. This dryer stagehas dry walls that are as warm, or warmer than the walls of theinitiator. The evaporator raises the temperature of the flow withoutintroducing additional water vapor, thereby reducing the saturation.

FIG. 2 is a graph of the calculated saturation ratio (top) and dropletdiameter (bottom) as a function of axial position for a system 100operated to produce saturations sufficient to activate 3 nm particles.In this graph the primary abscissa is the axial position divided by thevolumetric flow rate, in units of s/cm2. The secondary abscissa showsthe corresponding residence time when utilizing a cylindrical geometrywith an internal diameter of 2.5 mm.

Shown in FIG. 2 is the evolution of the saturation ratio and dropletdiameter along several radial trajectories, from the center line(r/R0=0) to the halfway point (r/R0=0.5), where R0 is the radius of thetube. The water vapor supersaturation is created as a result of thetransport of water vapor from the warm, wetted walls into the colderentering flow. Because the water molecule is smaller than theconstituent molecules in air, the water vapor diffuses more quickly thanthe sensible heat, creating a region of water vapor supersaturation.This process produces controlled, calculable supersaturations. Themaximum supersaturation occurs along the centerline, which handles themaximum of the flow.

With reference to FIG. 2, note that the maximum supersaturation occursat, or just past the exit of the initiator. This is because the watervapor that contributes to the supersaturation requires time, andtherefore distance, to reach the centerline. Moreover, most of thedroplet growth occurs in the evaporator section, and continues until thesaturation ratio drops below 1. Once the saturation ratio drops below 1,the particles will start to evaporate. By truncating the warm wet-walledsection the added water vapor is reduced, without affecting the peaksupersaturation, or activation size. Note also that the time requiredfor water evaporation for the non-hygroscopic particles of this model isabout the same as for their growth. For the narrow tube diameter ofthese calculations (ID=2.5 mm) the residence time for droplet growth andevaporation (for non-hygroscopic particles) is just 0.04 seconds.

FIG. 3 shows an alternate configuration of the condensation-evaporator102 that incorporates active removal of water vapor. The purpose is tolower the water content of the air flow to allow the temperature of theexiting flow to be lowered, without producing condensation. In oneimplementation, this system adds a cooled wall section 150 to removewater vapor through condensation to the walls, and it adds a warm,dry-walled section 160 to restore the flow temperature. Other methodsfor removing water vapor commonly known in the field could be used, suchas providing dessicant at the walls, or sheathing the flow with a dryair flow.

FIGS. 1 and 3 illustrate the ion source as positioned up-stream from thecondensation evaporator, but in alternative embodiments the ions may beintroduced between the initiator (130) and evaporator (140).

FIG. 4A illustrates a system condensation-evaporator for thenanoparticle charger, consisting of the inlet into which the particleladen flow and charging ions are introduced, and the conditioner,initiator, evaporator, water removal and temperature recovery sectionswhich enlarge and then evaporate the particles.

FIG. 4A shows the construction of a condensation-evaporator followingthe multistage approach of FIG. 3. FIG. 4B is a cross sectional viewalong line A-A in FIG. 4A. The particle-laden flow passes through an ionsource (not shown), and enters the condensation-evaporator at inlet 410.From there it passes through the five sections of thecondensation-evaporator, namely the conditioner 420, initiator 430,evaporator 440, water removal 450, and temperature recovery 460. Thisspecific system was designed for an air flow rate of 4.5 L/min. Itconsists of three parallel tubes (two tubes 412, 414 are illustrated inthe cross section of FIG. 4B), each with an inner diameter ofapproximately 5 mm. The lengths of the conditioner 420, initiator 430,evaporator 440, water removal 450 and temperature recovery 469 sectionsare, respectively, 220 mm, 85 mm, 125 mm, 125 mm and 85 mm. Theconditioner 420 and water removal section 450 are cooled by means ofthermal electric devices. The initiator 430, evaporator 440 andtemperature recovery 460 are heated either by contact film heaters orsmall heater cartridges. These temperatures are monitored usingthermistors or thermal couples, and controlled using standardlyavailable on-off or PID temperature controller circuitry.

FIG. 4B is a cross sectional view, showing two of the three tubesthrough which the flow passes. The walls of sections 420 and 430 arelined with a wick, constructed by rolling a piece of filter paper. Inthis construction, one wick spans both the conditioner and initiatorsections. This wick is wetted by means of a small reservoir of water 470located at the bottom of the initiator section 430. The lower end of thewick is mounted on a short standpipe that prevents the water fromflowing through the wick and into the flow. A small thermistor is usedto sense the water level, and triggers a water fill valve, which therebymaintains a fairly constant water level in the reservoir. The evaporator440 has no wick, and has warm, dry walls. This is followed by the waterremoval section 460, which again has a wick in each tube to absorb thewater that condenses from the air flow onto the walls. Each of the waterremoval wicks is mounted on a fluted standpipe which communicates withthe small cavity 480. A small suction flow in the range of 2% to 10% ofthe total flow is connected at the base of the water removal standpipesto removes the condensed water from the system. The three flows are thenjoined in the section 460 tail piece which is heated to providetemperature recovery.

FIG. 5A shows the typical wall temperatures used in the operation andtesting of the system of FIG. 4A, 4B. In this graph the axial position zis normalized with respect to the characteristic diffusion lengthz0=Q/D, where D is the diffusion coefficient for water vapor and Q isthe volumetric flow rate. The axial positions corresponding to each ofthe five sections (conditioner, initiator, evaporator, water removal andtemperature recovery) are labeled. As indicated, the conditioner spansthe axial evaporator extends over 0.04<z/z0<0.21, water removal over0.21<z/z0<0.32 and temperature recovery to 0.32<z/z0<0.4. The walltemperatures of the conditioner and initiator are nominally set at 2° C.and 35° C. respectively, but the model also includes short temperatureramps from the ambient conditions and between the successive sections.For reasons of construction, the evaporator has two temperature regions,one at the initiator temperature of 35° C. (corresponding to thestandpipe) and a longer one at 45° C. The water removal and temperaturerecovery sections are at 8° C. and 28° C.

FIG. 5B shows, for the scenario of FIG. 5A, the saturation ratioscalculated for flow trajectories along the centerline (solid line,r/Ro=0) and at a radial position equal to 70% of the tube radius(r/Ro=0.7, dashed line) where r is the radial position, and Ro is thetube radius. For this geometry and these operating conditions themaximum saturation ratio value is about 1.4, which is sufficient toactivate the condensational growth of particles roughly 5 nm indiameter. Because of the energy associated with surface tension, theequilibrium vapor pressure over the curved surface of the droplet ishigher than over a flat surface of the same chemical composition, thusrequiring greater than 100% RH values to initiate the condensationalgrowth. This dependence varies as the inverse of the particle diameter,as described by the Kelvin relation. As shown in FIG. 5B, the axialposition of the maximum saturation depends on the radial position, withthose trajectories closer to the tube wall achieving their maximumsooner (i.e. at lower axial positions) than along the centerline. Thisis typical of convective diffusion, where the transport of water vaporfrom the warmed walls of the initiator to the centerline takes moretime, due to the longer transport distance, and this greater timecorresponds to a larger axial position. In fact, with this configurationthe centerline saturation ratio reaches its maximum in the evaporatorsection.

FIG. 5C shows, for the scenario of FIG. 5A, the dew point valuescalculated for flow trajectories along the centerline (solid line) andat the radial position equal to 70% of the tube radius (dashed line).Although the relative humidity exiting the evaporator along thesetrajectories is between 4%-50%, the dew point is about 23° C. This meansthat the water content is sufficient to produce condensation if the flowis cooled to a typical ambient or room temperature of 20° C. For manyapplications it is desired to reduce this dew point, so that thedownstream components do not need to be heated. This is accomplishedthrough the water removal section, which has cooled walls. Byappropriately selecting the length of this section, and relying on thefast diffusion of water vapor, it is possible to reduce the dew point toaround 13°-14° C., without saturating the flow (except at the wallsthemselves).

FIG. 6 is an experimental configuration used to measure the chargingefficiency and charge distribution produced by a condensation-evaporatornanoparticle charger, and shows the differential mobility analyzer (DMA614) used to size-select the monodisperse test particles, thenanoparticle charger consisting of the bipolar ion source andcondensation-evaporator, and the scanning mobility particle sizingsystem (SMPS 630) used to measure the resulting distribution in particlemobilities.

The efficacy of the condensation-evaporator nanoparticle charger wastested using a bipolar ion source coupled to the inlet of thecondensation-evaporator, as shown in FIG. 6. As is commonly establishedaerosol technology, testing was done with size-classified, monodisperseparticles. Particles spanning a range of particle sizes are generatedusing an aerosol source 610, such as a furnace or atomizer. Thesepolydisperse particles pass through a bipolar ion source 612, where theyacquire a steady-state, or equilibrium electrical charge distribution.As is well known in the field, this bipolar ion source provides bothnegatively and positively charged particles (in addition to theuncharged particles), but minimizes the number of multiply chargedparticles. Next the particles are size-classified using a differentialmobility analyzers (DMA) 614, which selects particles of a singlepolarity and electrical mobility based on the transit time and electricfield within the mobility analyzer. These monomobility particles werethen passed through a nanoparticle charger consisting 620 of a softX-ray bipolar ion source 622 (TSI Model 3087, Shoreview, Minn.) and acondensation-evaporator 100 or 102. The electrical mobility of thepositively charged particles exiting the nano-charger was measured usinga second mobility analyzer 630 operated in scanning mode, commonlyreferred to as an SMPS. The mobility distribution of the positivelycharged particles output by the nanoparticle charger provides the totalnumber of positively charged particles as a function of the number ofcharges per particle. The overall efficiency for charging is determinedby comparison to SMPS measurements upstream of the nanoparticle charger,as measured on the “bypass” line of FIG. 6. For comparison toestablished bipolar charging methods, measurements are also made usingthe bipolar ion source alone, without the condensation evaporator, asshown by the “bipolar reference” line of FIG. 6.

The first set of experiments used a system of like that shown in FIG. 1,where the ion source was coupled to a condensation-evaporation systemwithout a water removal system. The bipolar ion source was provided bythe soft-Xray source (TSI Model 3087). The condensation evaporator 100used a conditioner, initiator, and evaporator measuring 116 mm, 70 mmand 112 mm in length, respectively. The system had three parallel tubes,with an inner diameter of 0.18 inches. No water removal section wasemployed. The wall temperatures were 5° C., 26° C. and 32° C. for theconditioner, initiator and evaporator, respectively. The system wasoperated at 4.5 L/min air flow with an initiator-dryer residence time of140 ms.

FIG. 7A shows the mobility distribution output by the nanoparticlecharger of the configuration shown in FIG. 1 using a bipolar ion source,when presented with a test aerosol is centered at 25 nm. Singly charged25 nm particles appear at a mobility size of 25 nm, while multiplycharged particles, being more mobile, appear at smaller mobilitydiameters. The mobility distribution for simple, bipolar charging isalso shown.

FIG. 7A shows the mobility distribution measured for a input particlesize centered at 25 nm that has passed through the nano-charger (solidline), or through a traditional bipolar ion source (dashed line). Theabscissa of this graph expresses the measured mobility as the diameterof a singly charged particle of the same mobility. Singly chargedparticles of the selected upstream DMA size appear at that same mobilitysize in the downstream size distribution. Doubly charged particles,being more mobile, appear at a smaller mobility size. For particles thatpassed through the bipolar ion source only, a unimodal mobility peakcentered at the input particle size of 25 nm is seen, and the fractionof those particles sampled that are positively charged is 11%. This isas predicted by the Fuchs relations for particle charging. For particlesthat passed through the nano-charger, consisting of the same bipolar ionsource followed by the condensation-evaporator, the fraction ofparticles carrying a positive charge increases to 33%. Of these, justover one-half of the charged particles carry a single charge, aboutone-third are doubly charged, and a few are triply charged. Because theion source used is bipolar, an approximately equal number of particleswill be negatively charged, bringing the total charged fraction to about60-65%.

FIG. 7B shows results for the same system for an input particle sizecentered at 10 nm. As for the 25 nm test particles, those particles thatpassed through the bipolar ion source only, exhibit unimodal mobilitypeak centered at the input particle size of 10 nm, indicating thepresence of singly charged particles only. However, at this smaller sizethe fraction of those particles sampled that acquire a positive chargeis ˜5%. This is significantly lower than at 25 nm, and is consistentwith theoretical predictions for bipolar charging. Those particles thatpassed through the nano-charger, again consisting of the same bipolarion source followed by the condensation-evaporator, the fraction ofpositively particles charged increases to 30%. This is close to thevalue obtained at 25 nm. As before, just over one-half of the chargedparticles produces by the nano-charger carry a single charge, and theremaining are mostly doubly charged. While the efficiency for acquiringcharge using bipolar ions decreases dramatically for the smallerparticles, with the bipolar ions coupled to the condensation-evaporator,the charging efficiency is essentially the same for both input particlesizes, with a value near 30%. Additionally, most charged particles carryjust one net charge.

FIG. 8A shows the mobility distribution output by the nanoparticlecharger of the configuration shown in FIG. 4 using a bipolar ion source,when presented with a test aerosol is centered at 20 nm. Singly charged20 nm particles appear at a mobility size of 20 nm, while multiplycharged particles, being more mobile, appear at smaller mobilitydiameters. The mobility distribution for simple, bipolar charging isalso shown. FIG. 8B shows the mobility distribution obtained with thesystem of FIG. 8A for an input aerosol centered at 10 nm. For the datashown the operating temperatures were 2° C., 35° C., 45° C., 8° C. and28° C. for the conditioner, initiator, evaporator, water removal andtemperature recovery stages. The flow through the system was 4.5 L/min,and the sampled air was dry, at ˜25° C.

FIG. 8B shows the mobility distribution output by the nanoparticlecharger of the configuration shown in FIG. 4 using a bipolar ion source,when presented with a test aerosol is centered at 10 nm. Singly charged10 nm particles appear at a mobility size of 20 nm, while multiplycharged particles, being more mobile, appear at smaller mobilitydiameters.

FIGS. 8A and 8B show results for the system of FIG. 4, where a waterremoval and temperature recovery stages have been added to the system.With the additional stages, it is possible to operate at a largertemperature for the initiator, without producing condensation once theexiting flow is returned to room temperature. This higher temperaturedifference between the initiator and the conditioner provides a highersupersaturation, which activates to smaller particle sizes and producessomewhat larger droplets. The result is a somewhat higher chargingefficiency. As shown in FIGS. 8A and 8B, with this configuration theefficiency for placing positive charges onto the particles is close to40% at both 10 nm and 20 nm. Approximately 60% of those charged carry asingle net charge. By comparison, the efficiency for bipolar charging is4% at 10 nm, and 8% at 20 nm.

In summary, a condensation-evaporation system has been designed for usewith an ion source to provide more efficient charging of nanometer-sizedparticles than is possible through direct exposure to an ion source. Intests with a bipolar ion source, this nano-charger provides chargingefficiencies of the order of 30% to 40% for a single polarity. An equalfraction of particles of the opposite polarity will be charged, leadingto a total charge fraction of 60-80%. Of those particles that arecharged, approximately 60% carry a single net charge, with the remainingfraction carrying two or more charges. The fraction of charged particlesis found to be independent of the input particle size. Yet highercharging efficiencies may be achieved if the bipolar ion source werereplaced with a unipolar ion source, however this may lead to a higherproportion of more highly charged individual particles.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A particle charging apparatus, comprising: an ionsource; and a container having at least one wick to provide wetted wallsinto which an air flow is introduced in a laminar manner, said containerhaving at least a first section, a second section and a third section,the first section including the wetted walls at a first temperature T1,the second section adjacent to the first section and having the wettedwalls at a second temperature T2 greater than the first temperature T1,the third section adjacent to the second section and having dry wallsprovided at a temperature T3 greater than T2, the temperature T3 beinghigher than a condensation point of the flow entering the third section.2. The particle charging apparatus of claim 1 wherein the air flowpasses through both the ion source and the container.
 3. The particlecharging apparatus of claim 1 wherein the ion source is positioned tooutput the air flow to an input to the first section.
 4. The particlecharging apparatus of claim 1 wherein the ion source is bipolar.
 5. Theparticle charging apparatus of claim 1 wherein the ion source isunipolar.
 6. The particle charging apparatus of claim 1 furtherincluding a fourth section of the container having walls at atemperature T4 cooler than T2 and T3.
 7. The apparatus of claim 6further including a fifth section of the container having dry wallsoperated at a temperature T5 greater than the temperature T4.
 8. Ananoparticle charger apparatus, comprising: an ion source; a containerhaving at least one wick to provide wetted walls into which an air flowis introduced in a laminar manner, said container having at least afirst section, a second section, a third section, and a forth sectionthe first section including the wetted walls at a first temperature T1,the second section adjacent to the first section and having the wettedwalls at a second temperature T2 greater than the first temperature T1,the third section adjacent to the second section and having dry wallsprovided at a temperature T3 greater than T2 and the temperature T3being higher than a condensation point of the flow entering the thirdsection, the fourth section operated at a temperature T4 selected toremove water vapor from the flow.
 9. The particle charging apparatus ofclaim 8 wherein temperature T4 is warmer than T1 and cooler than T2 andT3.
 10. The apparatus of claim 8 wherein the air flow includes a carriergas and a particle laden flow.
 11. The apparatus of claim 10 wherein thecarrier gas is air.
 12. The particle charging apparatus of claim 8wherein the air flow passes through both the ion source and thecontainer.
 13. The particle charging apparatus of claim 12 wherein theion source is positioned to output the air flow to an input to the firstsection.
 14. The particle charging apparatus of claim 13 wherein the ionsource is bipolar.
 15. The particle charging apparatus of claim 13wherein the ion source is unipolar.
 16. A method of electricallycharging particles, comprising: providing a container having at leastone wick to provide wetted walls into which a particle laden flow may beintroduced, having at least a first section, a second section adjacentto the first section and a third section adjacent to the second section;passing the particle laden flow through an ion source; introducing theparticle laden flow into the container in a laminar manner; andoperating the first section with wetted walls at a first temperature T1,the second section at a second temperature T2 greater than the firsttemperature T1, and the third section with dry walls at a temperature T3greater than T2, and the temperature T3 being higher than a condensationpoint of the flow entering the third section.
 17. The method of claim 16wherein the step of passing occurs prior to the step of introducing. 18.The method of claim 16 further including providing a fourth sectionadjacent to the third section and operating the fourth section at atemperature cooler than T2 and T3.
 19. The method of claim 18 furtherincluding providing a fifth section adjacent to the fourth section andoperating the fifth section at including a fifth section at atemperature T5 greater than the temperature T4.